Sense mode for `cable_probe_membrane_voltage`

[schmitts]

cable_probe_membrane_voltage interpolates/extrapolates currently the voltage based on the voltages of the CVs. This may lead to unexpected results, cf. #1533. A "mode" should be introduced that allows to switch to sensing the voltage of the CV on which the probe resides. cable_probe_membrane_voltage_cell is not sufficent as one does not always need the voltages on all CV. The "mode" should really be a mode and not only a flag as there might be more than two modes in the future.

[thorstenhater]

I can attest that I fell for this hard and spent a week checking the numerics of Arbor earlier this year.